Like Extra Utilities and Railcraft do. Would make it not an issue of "upgrade and drop rarely-updating good minor mods or not upgrade major ones".

I wouldn't have thought it was even possible to do so, had they not managed it. Even now, I have no idea how, and as both mods are closed source I have no way of working it out. Maybe @aidancbrady has some idea?

Railcraft is opensource: https://github.com/CovertJaguar/Railcraft
You just might need to check the license

So it is. I must have missed that happening. We'll potentially look at getting that working at some point then, but I've not got any time at the moment.

Gendustry also allows for both versions of fluid handling and is licensed under MMPL, basically saying "do whatever": https://github.com/bdew/gendustry

yeah hope it is done til all mods have updated past the fluid change so everyone can make use of the updated bug fixed version yet during that time

Low priority, mods should update to the new forge fairly soon. In the meantime you can just use an older mekanism release.